1. Field of the Invention
The invention in general relates to trimming or adjusting components of an electronic circuit so that the circuit performs within specifications, and more particularly to an automated process and apparatus for trimming components while the circuit is turned on or active.
2. Statement of the Problem
The process of trimming elements of an electronic circuit to bring the circuit within predetermined specifications is well known. For example, capacitors, resistors, inductors and other elements may be adjusted to cause the circuit to perform within set specifications. As other examples, electronic elements may be trimmed to cause a circuit to oscillate at a specified frequency, to set off an alarm at a specified threshold value, to amplify with a particular gain, or to amplify with a particular gain within a certain frequency range. The trimming may be done by adjusting the values of variable capacitors, resistors, or inductors with a set screw. Such manual adjustments can vary over time, thus more reliable methods of adjustment were developed, such as substituting one element for another, or adding or subtracting elements in a circuit. For example, a fuse may be melted to remove one or more parallel resistors from a circuit to change the resistance of a portion of the circuit. As electronic circuits have become more sophisticated and capable of operating with higher degrees of accuracy, more sensitive ways of trimming circuits lave been devised. One such method is the trimming of resistors or capacitors with lasers. This allows a tiny portion of the resistor or capacitor to be removed, thereby changing the electronic parameters of the circuit by a commensurate small amount. Since many circuit parameters vary with temperature and other factors that may be different when a circuit is turned on than they are when the circuit is turned off, when high accuracy is required in circuit specification, the trim is often done while the circuit is turned on. Primming when the circuit is turned on is called "active trim".
Electronic circuits that are used to test other electronic circuits or to measure electronic values in other circuits must operate very accurately within predetermined specifications, since the accuracy of the circuits tested depends on the accuracy of the test instrument. Connections between the circuit to be tested and the test instruments, such as oscilloscopes, are generally made with an instrument called a test probe. The requirements for accuracy of operation within the specification for test probes are extremely high, since any inaccuracies in the test probe will be passed through to, and may be multiplied by, the test instrument. The invention will be illustrated as applied to the trimming or adjustment of the operating parameters of an electronic test probe.
A test probe is essentially an impedance buffer, that is, a circuit with an output having a significantly different impedance than the impedance of its input. Generally, the voltage or other electrical parameter on the output of the test probe follows the voltage or other electrical parameter applied to its input. In addition, the nominal voltage at the input, that is, the voltage when the probe is not being applied to a circuit to be tested, is generally a zero voltage so that the probe does not apply any voltage to the circuit node to which it is applied. Further, the impedance at the input is as high as possible to prevent the test probe and test instrument to which it is connected from drawing significant current or otherwise significantly altering the electrical parameters on the node to be tested. The impedance of the output is generally a standard value, such as 50 ohms, to which the test instrument is designed to couple. Early test probes comprised simple conductors, such as a wire, and a few passive components, such as resistors, to provide an impedance buffer. Such passive test probes are adequate for connecting test equipment to circuits with DC or relatively low frequency electrical cycles. Present-day high frequency circuits require active probes, that is probes with active circuit elements, such as transistors, driven by a probe power source. Since the difference in the physical and electronic properties of such active elements when they are off and when they are on can be relatively large compared to the inaccuracies tolerable in an active probe, it is highly desirable that a very accurate method of active trim be available for such probes.
The frequencies of electrical signals that an active probe are called on to transmit to a test instrument can vary from zero, in the case of DC signals, to several gigahertz, in the case of extremely fast digital circuits. Thus it is essential that such active probes have a response that is flat to a high degree of accuracy across a wide range of frequencies, generally called a band width (BW). As a result there is a need for a method and apparatus for actively trimming an active probe over a wide band width.
The problems of trimming electrical parameters at DC and high frequencies are considerably different. It is possible to measure accurate and stable electrical quantities, such as voltage, at the DC level. Thus the problems of trimming at the DC level relate to the need for obtaining and reliably reproducing absolute measurements. However, in the AC case, it is significantly harder to generate an accurate, stable AC voltage, whether it is a sine wave or step pulse. And accurate AC measurements are extremely hard to perform at very high frequencies. Thus there is a need for an active trim method and apparatus that produces results at the DC level that are consistent with results at high frequencies.
State-of-the-art active test probe designs are relatively complex and generally include several subcircuits and/or functions that must be individually trimmed. For example, the probe may contain a DC amplifier and an AC amplifier each of which must be adjusted to be flat. The specifications may require a specific input capacitance and a specific impedance attenuation. These circuits and functions are not independent, so that trimming one will generally affect the other. Thus there is a need for a method of trimming electronic circuits that permits accurate trimming to simultaneously meet a plurality of circuit constraints.
At the high accuracies of specifications and measurement which are applicable to active probes, factors that are insignificant with other circuits become significant. For example, the probe is sold and used within a probe housing, however, active trim must be done with the housing removed, since the elements such as resistors and capacitors that must be trimmed cannot otherwise be reached. The difference in electrical parameters of the probe within and outside of the housing are larger than the error tolerance of the trim function. Thus there is a need for a method and apparatus for active trim of an electronic circuit that can reproduce the actual conditions of use of the electronic circuit within a high degree of accuracy.
Electronic instruments such as probes are generally made in significant numbers. Manual testing of each probe can be very expensive and add considerably to the cost of the probe. Thus there is a need for a trim system that overcomes the above problems and is also performed automatically.
3. Solution to the problem
The present invention solves the above problems by trimming the DC circuit to an absolute voltage using a precision voltage source and a precision voltmeter. The trim of the AC circuit is then referenced to the trim of the DC circuit.
The DC trim is done by laser trimming a resistor in the probe DC amplifier circuit until the desired absolute probe voltage attenuation is reached.
The referencing is done by first obtaining calibration factors that reference the AC voltages to a DC voltage, then adjusting the AC responses to match the calibration factors. The calibration factor is obtained by sending a step voltage through a circuit with a known response equivalent to the desired response of the probe circuit. The difference between the voltage at 80 nsec, where the step is essentially AC, and the voltage at 1.4 .mu.sec, where the step is essentially DC, provides a first calibration factor. The difference between the voltage at 3 nsec and 80 nsec provides a second calibration factor that carries the calibration out to very high frequencies.
The high frequency AC trim is performed by applying the step voltage to the probe input, connecting the output to an averaging oscilloscope, and laser trimming a resistor in the AC amplifier until the difference between the 80 nsec voltage and the 1.4 .mu.sec voltage equals the first calibration factor.
The input compensation trim is performed by applying the step voltage to the probe input, connecting the output to an averaging oscilloscope, and laser trimming a capacitor in the input compensation circuit until the difference between the 3 nsec voltage and the 80 nsec voltage equals the second calibration factor.
A step scan laser trim is used in the DC trim and an L-cut laser trim is used in the AC trim. These novel trim methods allow the trim to be performed rapidly while the output voltage is far from the target voltage, and to gradually be trimmed more slowly and accurately as the desired voltage value is reached.
During the trim the probe circuit board is enclosed in a trim housing that replicates the effect of the probe housing on the probe circuit. Ports in the trim housing allow the laser beam to reach the components to be trimmed.